1. Field of the Invention
This invention relates to color filters having nematic liquid crystal cells, and to compensating for the chromaticity of the nematic cells and utilizing fast transitions of the nematic cells in switching the color filters.
2. Background of the Invention
Switched-polarizer-filters (SPFs) have one or more stages, each stage consisting of a color polarizer and a two state polarization switch. The color polarizer provides different transmission spectra along orthogonal polarization axes, and the polarization switch is a switchable analyzer which selects which of the two polarizations is transmitted. The SPF is intrinsically binary tunable, such that each filter stage permits switching between two colors. Stages are cascaded in order to provide additional output colors.
Color polarizers used in SPFs are generally dye-type pleochroic color polarizing filters (for example in U.S. Pat. No. 4,582,396 to Bos, U.S. Pat. No. 4,416,514 to Plummer, U.S. Pat. No. 4,758,818 to Vatne and U.S. Pat. No. 5,347,378 to Handschy). Dye-type pleochroic color polarizers are films that function as linear polarizers in specific wavelength bands. They are formed by doping a polymer with long-chain pleochroic dyes. Incident white light polarized along one axis is fully transmitted, but is selectively absorbed along the orthogonal axis. For instance, a cyan color polarizer functions as a linear polarizer by absorbing the red along one axis. A color polarizer that passes a primary color (either additive or subtractive) along each axis can be formed as a composite consisting of two films with crossed axes. Colors are typically selected using crossed complementary color (eg. red/cyan) polarizer films coupled with a switchable polarizer. A full color device can comprise five polarizing films (one neutral), and two switching means.
Recently, polarizer-retarder-stack (PRS) color polarizers have been described (U.S. patent application Ser. No. 08/447,522, filed May 23, 1995, and now U.S. Pat. No. 5,751,384, which is incorporated by reference herein in its entirety). The PRS comprises a linear polarizer in combination with a stack of retarders. The number of retarders in the stack and the retardances and orientations of the retarders are selected such that an additive primary spectrum is transmitted along a first polarization axis and the complementary subtractive primary spectrum is transmitted along the orthogonal polarization axis.
The polarization switch in a SPF can be a liquid crystal (LC) cell in combination with a static polarization analyzer. The switch optimally provides neutral polarization switching. Unfortunately, the performance of these filters is significantly degraded by the chromatic nature of the liquid crystal active element. In general, liquid crystal cells accurately switch between orthogonal polarizations in only one spectral region. Outside of this region the filters are subject to leakage of the blocked bands and reduced throughput of the pass-band.
Smectic liquid crystal polarization switches generally comprise a linear polarizer and a liquid crystal half-wave retarder with fixed retardance and rotatable orientation. Rotating the optic axis of the liquid crystal cell between 0.degree. and 45.degree. with respect to the polarizer switches the polarization between two orthogonal axes. Achromatic smectic liquid crystal polarization switches have recently been described (U.S. patent applications Ser. No. 08/419,593, filed Apr. 7, 1995, now U.S. Pat. No. 5,658,490, and Ser. No. 08/549,963, filed Oct. 30, 1995), and can be used to provide high quality color filters.
Nematic liquid crystal polarizations switches generally comprise a linear polarizer and a nematic liquid crystal cell with fixed orientation and variable retardance. The achromatic polarization switches developed for smectic liquid crystal cells cannot be employed with nematics, leaving nematic liquid crystal color filters beset with leakage and low throughput.
A second drawback to nematic liquid crystal filters is their slow switching speed. Nematic liquid crystal cell typically take a few hundred microseconds to realign in response to an applied electric field and much longer, several milliseconds to reset to the unenergized state. Because of the slow response time, the rate at which the filter can sequence through the three primary colors is limited, which in turn limits the rate at which subframes can be acquired or displayed for camera or display applications.